Hetero atoms

In the PDB data file format for macromolecular models, each atom is designated either ATOM or HETATM (which stands for hetero atom). ATOM is reserved for atoms in standard residues of protein, DNA or RNA. HETATM is applied to non-standard residues of protein, DNA or RNA, as well as atoms in other kinds of groups, such as carbohydrates, substrates, ligands, solvent, and metal ions.